Project Summary/Abstract: Pediatric high-grade gliomas (pHGG) account for the most cancer-related deaths in children under the age of 19 years old. Recent advances in our understanding of pHGGs have demonstrated that pHGGs drastically differ from their adult counterparts in terms of genetic and epigenetic alterations, suggesting that they may also differ in the constituency of their tumor microenvironment. Despite our growing understanding of these tumors, clinical trials for pHGG are often based on highly ineffective adult therapies while ignoring the inherent biological differences between the two entities. Further, the efficacy of targeted therapies has been limited due to targeting deregulated mechanisms within tumor cells only. It is now known that tumor associated macrophages (TAMs) can make up to 30-40% of the total tumor cell mass in adult high grade gliomas, and play important roles in immune suppression and tumor promotion. This begs this question of whether targeted therapies fail due to the adaptive tumor-microenvironment. It also raises the question of whether pHGGs are similar in the constituency and behavior of TAMs despite containing different genetic and epigenetic landscapes. To uncover the role of TAMs in pHGG we utilize RCAS/tva, a somatic cell type specific gene transfer system in which we administer RCAS-PDGFA or RCAS-PDGFB to stimulate PDGF receptor activation in Cdkn2a or Tp53 deficient mice. The most common genetic alteration in human pHGG is PDGFR amplification followed by CDKN2A loss and TP53 mutation. It is known that PDGFA can only bind and activate PDGFRA on tumor cells while PDGFB can bind and activate both PDGFRA and PDGFRB found on tumor and stromal cells. Our preliminary data suggest that PDGFB-driven tumors have an increased infiltration of inflammatory monocytes, and we hypothesize that is attributable to the additional activation of the stromal population by PDGFB and the elevated expression of CCL2. Further, PDGFB-driven mice have a significantly lower median survival, indicating that the TAM population may be associated with increased malignancy. It is currently unknown what percentage of these cells are monocyte derived versus resident microglia. The molecular identity and location of these TAMs are also unknown. To address these fundamental yet highly important issues, we plan to uncover the cellular identity, composition, and location of TAMs in pHGG utilizing flow cytometry, RNA sequencing, and two-photon microscopy. Additionally, we plan to determine if inhibiting the CCL2-CCR2 inflammatory signaling axis can inhibit TAM infiltration and prolong survival in pHGG. These studies will lay the foundation for the future development of more efficacious immunotherapies targeted at pHGG.